I think it was later clarified, that it meant retrospect deletion.
Before the change the wayback machine had the problem that when someone new bought the domain and restricted the access via robots.txt, the whole archive of the site of the previous owner was deleted.
This was not only bad because of accidental deletion, but people could even intentionally destroy parts of the archive by buying old domains.
For just denying access to the archive bot have a look at the "noarchive" flag.
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